Move plugins to manifest, pin Docker version, add Makefile

- Add plugins.txt listing all plugins for reproducible installs
- Add Makefile with setup/start/stop/install-plugins targets
- Remove user/plugins/ from git tracking
- Pin Docker image to 1.7.49.5-ls244

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/admin/themes/grav/scss/vendor/bourbon/settings/_prefixer.scss

@@ -1,6 +0,0 @@
-// Variable settings for /addons/prefixer.scss
-$prefix-for-webkit:    true !default;
-$prefix-for-mozilla:   true !default;
-$prefix-for-microsoft: true !default;
-$prefix-for-opera:     true !default;
-$prefix-for-spec:      true !default; // required for keyframe mixin

plugins/admin/themes/grav/scss/vendor/bourbon/settings/_px-to-em.scss

@@ -1 +0,0 @@
-$em-base: 16px !default;